1. Field of the Invention
This invention relates to apparatus and method for forming a particulate layer on a substrate surface from a gaseous suspension of particulate, thermoplastic material, and more particularly to apparatus and method utilizing electrostatics for forming a particulate layer on a substrate surface.
2. Description of the Prior Art
Considerable interest exists in electrostatic powder deposition systems for forming a particulate layer on a substrate surface from a gaseous suspension of particulate, thermoplastic material. These systems are being considered for use in forming many different types of layers on various types of substrates. For example, these systems are being considered for use in forming continuous webs of various thicknesses on a continuous substrate, such as a release paper substrate. One suitable release paper substrate is sold under the trademark "Transkote" by S. D. Warren Company, a division of Scott Paper Company. When the substrate is release paper the electrostatically deposited layer of particulate, thermoplastic material is fused to form a self-supporting continuous and uniform web which is removed from the substrate. Such a self-supporting web has many uses; for example, as a substitute for leather in products such as shoe uppers, upholstery and the like.
The substrate can be a discrete metal mold, or other suitable mold known in the art, having a cavity or cavities which conform to the configuration of a discrete article to be formed. For example, bottles, cartons, food trays, containers, and the like can be formed directly in such a discrete mold.
The substrate can also be a discrete article of manufacture such as a pipe, tube, tool handle and the like upon which a protective layer of fused, particulate material is required. The fused particulate layer is not removed from a discrete article when it is intended to function as a protective layer.
In most applications in which a layer of particulate material is formed on a substrate surface it is an important objective that the layer be substantially uniform and continuous. The prior art methods and apparatus have not been entirely satisfactory in accomplishing this objective.
Several types of feed systems have been utilized to feed an air suspension of particulate, thermoplastic material to spray means through which the air suspension of particulate material is directed onto a substrate surface. For example, an air suspension of particulate material has been contained in a reservoir and fed through a metering control valve to spray nozzles for coating pipes, as disclosed in U.S. Pat. No. 3,439,649, issued to Probst et al. Also, particulate material has been directed to spray nozzles through vibratory feeders, as disclosed in U.S. Pat. No. 3,155,545, issued to Rocks et al; and from fluidized beds, as indicated in U.S. Pat. No. 3,498,540, issued to Adams. The above feed systems have not been entirely satisfactory in directing a substantially constant volume of properly-sized particulate material to spray nozzles to aid in the formation of a substantially uniform and continuous layer of particulate material on a substrate surface.
Many types of spray nozzles have been utilized in directing a gaseous suspension of particulate, thermoplastic material onto a substrate surface, as exemplified in U.S. Pat. Nos. 3,521,815 issued to Szasz; 3,498,540, issued to Adams; 3,476,319, issued to Thatcher and 3,155,545, issued to Rocks et al. These prior art spray nozzles have not been entirely satisfactory in creating and maintaining a substantially uniform concentration of the particulate material in the gaseous suspension as said suspension is directed through said spray nozzles onto a substrate surface. Specifically, these prior art spray nozzles have not provided sufficient mixing of the gaseous suspension of particulate material to create a substantially uniform concentration of said particulate material in the gaseous medium for deposition onto a substrate surface.
Prior art apparatus for applying layers of particulate material on a substrate have included various types of feeding mechanism for depositing the particulate material on a substrate. These feeding mechanisms include hopper-type feeders, as is exemplified in U.S. Pat. No. 3,168,603, issued to Seefluth; shaking devices, as exemplified in U.S. Pat. No. 3,192,294, issued to Streed et al; and stationary spray nozzles, as exemplified in U.S. Pat. No. 3,155,545, issued to Rocks et al and U.S. Pat. No. 3,355,309, issued to Bender et al. These prior art arrangements for applying particulate material have not been satisfactorily employed in forming particulate layers on extensive substrate surfaces, such as long panels and continuous substrate surfaces. These prior art arrangements for directing particulate material onto extensive substrate surfaces have required extremely slow formation speeds in order to supply a sufficient quantity of particulate material over the entire surface of an extensive substrate.
The use of a plurality of spray guns for coating an extensive work piece has been suggested in U.S. Pat. No. 3,402,697, issued to Kock. The Kock patent primarily is directed to forming a coating of paint, for example, on an extensive surface area, and suggests that the plurality of spray guns be mounted on a reciprocating support. At high coating speeds, this reciprocating support must be reciprocated at a relatively fast frequency, and the instantaneous change in direction of reciprocation may adversely affect the formation of a substantial uniform coating on said substrate surface.
U.S. Pat. No. 2,718,477 issued to Miller, suggests the coating of an extensive web of liquid atomization in which a flexible discharge member, such as a continuous sheet metal belt, has its opposite, substantially flat surfaces coated by stationary spray nozzles with the liquid material to be applied to the web. This arrangement could not be satisfactorily utilized in an apparatus in which an air suspension of particulate material, as opposed to a liquid suspension of material, is to be applied to an extensive substrate surface because the air suspension of particulate material would not adhere to the surfaces of the belt in the manner disclosed in the Miller patent.